Fourier transform infrared (FTIR) spectrometers are utilized in the analysis of chemical compounds. In these instruments, an infrared source provides a beam of infrared radiation having a band of infrared wavelengths which is passed into an interferometer, typically a Michelson interferometer, and is modulated before being passed through or reflected from the sample to be analyzed. The beam is then directed to a detector. The interferometer modulates the radiation received by it to provide an output beam in which many narrow ranges of infrared wavelengths are typically reduced or enhanced in intensity, with the affected range of wavelengths changing periodically over time. The time correlated output signal from the detector is analyzed by Fourier transformation to derive information on the characteristics of the sample. Examples of FTIR spectrometers and interferometers for such spectrometers are shown in U.S. Pat. Nos. 4,799,011, 4,847,878, 5,153,675, 5,883,712, 5,896,197 and 6,667,808.
Typical infrared sources used in infrared spectrometers use a source element formed of a loop of high resistance electrical conductor, such as silicon carbide, that is supplied with electrical power to be resistively heated to a high temperature that is typically in the range of 1,000° to 1,300° C. At these temperatures, the source element radiates a broad range of infrared light as well as visible light. As used herein, visible and infrared radiation will both be referred to as “light.” Generally, the intensity of both the infrared and visible light emitted by the source element is proportional (although not necessarily linearly) to the electrical power supplied to the source element. Commonly used source elements include igniters of the type used in furnaces and stoves. Infrared sources for spectrometers typically include an insulated enclosure surrounding the source element with an opening in the enclosure through which the beam of infrared radiation can exit in a desired direction. An example of an infrared source for infrared spectrometers with a fully enclosed source element is shown in U.S. Pat. No. 5,291,022, which is incorporated by reference. More commonly, the source element is exposed to the surrounding atmosphere.
To obtain consistent spectra from a spectrometer, it is desirable that the intensity of the output beam from the infrared source be relatively constant. Maintaining a constant output from the source is important to spectrometer performance because changes in energy output from the source are reflected in the spectra that are produced by the instrument. For example, a decrease in source energy that may occur between the time when a background spectrum is taken and the time when a sample spectrum is taken will produce a sample spectrum that is erroneously low in energy. Changes in source energy also produce a spectral shift in the instrument line shape due to the black body changes that occur with temperature, and this will also distort spectra produced by the spectrometer.
The source element is typically operated in air, and short-term changes in the surrounding air and purge gas flows within the spectrometer housing can cause short duration changes in the source temperature. Long-term changes in the source temperature occur as the source degrades due to chemical reactions which result from the high operating temperature of the source.
Conventional spectrometers have attempted to maintain the energy output of the source constant by supplying the source element with a constant voltage or current. However, over a long period of time, the output of a typical infrared source element for a given voltage or current input will decline, and most source elements have a non-linear relationship between supply current and element temperature so that control of the voltage or current supplied to the source element does not assure a constant light energy output from the source element over time. Furthermore, most infrared sources have finite lifetimes because of surface oxidation and other problems related to the high operating temperatures (1000° to 1300° C.). The infrared sources can thus burn out, sometimes without advance warning, requiring replacement of the source before further measurements can be done using the spectrometer.